What period of time could a less severe variant(s) have escaped into the wild before the more severe 'Alpha' variant gained traction? AFAIK, China, & others, we

What period of time could a less severe variant(s) have escaped into the wild before the more severe 'Alpha' variant gained traction? AFAIK, China, & others, were treating non-influenza, flu-like cases months before they cancelled Chinese New Year 2020. The timeline may not have been as short as a Stephen King novel and, perhaps, it was even prior to Oct/Nov 2019. My question is this: Are there any frequent/infrequent travelers to China familiar with non-asian visitors getting severe influenza(or other) infections when they 1st visit? I know one such traveler who spent 3 months in a "ghost room"(1) suffering from a lung infection back in early 2019. (1) Hospice; he reported it as called the 'ghost room' as the rest of the occupants were terminal cancer patients.